It's a difficult question. Most of the quests are good quests ( since they show how creative people can be ). I have not tried them all though, but of the Quests i have played, this was the best and the hardest ( In my oppinion ).

The Cursed book: by Malleus
StandingStonesV115: by Chasm
Avenger: by Foucter

As i said, I have not tried the new ones yet!

Torinth and The Graveyard is also very playable ( as far as I remember ).

Maybe you will have a look at my own Quest For Glory too ( It was expected that I would mention my own quest,right? ).....